Due to the social needs to save energy, recently, the decrease in the fuel consumption by automobiles is being extremely severely required. As for the properties of a tire, a further decrease in the rolling resistance is required in response to the requirement. As the method for decreasing the rolling resistance of a tire, the use of a material exhibiting smaller heat buildup as the rubber composition has been widely conducted although methods of optimizing the structure of the tire have also been studied.
To obtain a rubber composition exhibiting a small heat buildup, heretofore, many developments of technology on modifying rubber for rubber compositions using silica and carbon black as the filler have been conducted. In particular, the methods of modifying the interaction of fillers with active chain ends of diene-based polymers, which are obtained in accordance with the anionic polymerization using organolithium compounds, using alkoxysilane derivatives have been proposed as the effective method.
Many of these methods are applied to polymers which are easily provided with living active chain ends. However, rather few proposals are found for modification and improvement of cis-1,4-polybutadiene which is important, in particular, as the rubber for tire side walls and tire treads. Moreover, the sufficient effect of the modification on rubber compositions containing silica and carbon black has not always been obtained. In particular, it is the actual situation that almost no effect of the modification is obtained on the rubber compositions containing cis-1,4-polybutadiene and carbon black.
The conventional methods of the modification have a further problem in that cold flow is the major obstacle for the actual application since sufficient branching cannot be provided to the main chain and the effect of the modification inevitably decreases when partial coupling is made to overcome this problem.
On the other hand, it has been attempted that a conjugated diene polymer modified with a silane is obtained by the reaction of an alkoxysilane compound with active chain ends having a great cis-content which is obtained by using a rare earth catalyst. However, in accordance with this method, the increase in the Mooney viscosity due to the modification with the silane is marked in many cases although a great effect of improving the cold flow can be exhibited, and gel having a visible size is frequently formed in the separated copolymer.
Thus, improvements are required for this method from the standpoint of workability and properties.